Non-mourners standing for mourner’s kaddish?
Forbidden? Required? Optional?
Thank you for your help.
The minhag of the Ashkenazim is that it is preferable to stand during kaddish, however the minhag of the Sefardim is not to be makpid on this. However the Arizal used to stand by any kaddish that was said when he was already standing, such as kaddish after Shmona Esrei to continue standing. With the mourners kaddish the same rules should apply.
If a sefardi is in a place that the minhag is to stand he should do likewise as not to give an appearance that there are different minhagim ( lo tisgodidu).
Rema O:CH 56-1, M:B ibid 8, Ohr L’tzion Vol. 2- chap. 5-9.